This invention relates to needles for the delivery of liquids to human or animal subjects.
The standard needle used for subcutaneous or intramuscular injections is in the form of a hollow shaft provided with a sharpened open end. This type of needle has been found to be unreliable in ensuring accurate delivery rates, in particular when used in conjunction with an infusion pump or some other delivery device which depends for delivery on the magnitude of pressure applied to the liquid being delivered.
The lack of accuracy is thought to be due to the build up of pressure at the delivery point which opposes the driving pressure applied to the liquid.
Others have tried to combat the problem by designing needles which were intended to avoid the problem of pressure build up arising from the orifice becoming plugged by tissue on entry into the skin.
U.S. Pat. Ser. No. 4,413,993 (Guttman), U.S. Pat. Ser. No. 4,790,830 (Hamacher), and U.S. Pat. Ser. No. 4,838,877 (Massau) each disclose a hypodermic or intravenous delivery needle having one or more apertures located on the side of the shaft removed from the sharpened tip.
U.S. Pat. Ser. No. 2,748,769 (Huber) discloses a hypodermic needle having a curved or bent tip cut in a plane that extends along the side of the needle towards which the bend is made and thereby providing an orifice which is not plugged by tissue upon insertion into a subject, the curved surface being provided with an auxiliary delivery orifice which ensures delivery when the main orifice rests against a vein wall.
U.S. Pat. Ser. No. 3,076,457 (Copen) discloses a hypodermic needle having an aperture at the tip and also having an opening which extends along the side of the shaft for part of its length.
None of these needles provides a satisfactory solution to the problem of ensuring accurate delivery using a delivery device in which the delivery rate depends on the generation of pressure, because in each case, the problem which has been addressed has been the provision of an additional orifice which will not become plugged upon insertion or which will not rest against a vein wall. While these problems must be addressed in designing a needle for use with an automated drug delivery device, it has been found that these factors alone do not account for the major variations in delivery rate which occur when using a device which depends for delivery on pressurising a supply of drug.
It has been found, when conducting tests of devices which operate on the pressurised reservoir principle, that there are large variations in the resistance to delivery into the skin not only when comparing different subjects or different delivery sites on the same subject (which could be accounted for by differences in tissue composition at the delivery site), but also when the resistance to delivery is monitored over time during a single test at a single site.
If the resistance to delivery (i.e. the impedance to the flow of drug) varies while the pressure effecting delivery remains fixed, then clearly the delivery rate (which is dependent on the difference in pressure between the reservoir and the delivery site) will fluctuate. Clearly this has serious implications for the delivery accuracy of such devices.
The best results in subcutaneous delivery tests of this type are obtained when using a needle such as that disclosed by Huber or by Copen (see above), wherein the aperture has a greater external surface area than internal surface area. Without wishing to be bound by any theoretical explanation, it is conjectured that the major obstacle to ensuring accurate delivery rates is not that the orifice becomes plugged, but rather that the difficulty arises from poor absorption by the tissue adjacent to the orifice(s), and/or from variations in the structure of the tissue at the orifice. This means that the drug accumulates at the orifice and the pressure builds up until absorption commences, since a given surface area of tissue can only absorb a certain amount of drug at a given pressure. In cases where the external aperture surface area is increased relative to the internal aperture surface area, the absorption (by a larger tissue area) is correspondingly better and fluctuations are reduced.
Nevertheless, it is not practicable to replace conventional delivery needles with needles of the type disclosed by Huber or Copen because of the substantial difference in manufacturing cost involved. A conventional delivery needle can be formed by cutting hollow steel tubing to the correct length, and grinding or in some other way machining the cut end to provide the necessary sharpness for skin penetration. The thus formed tip is of course automatically provided with a delivery orifice, and the needle is inexpensive.
In order to manufacture a needle of the type disclosed in any of the prior art documents referred to above, the initial steps of creating a needle must first be followed, and then the additional holes or orifices must be machined into the side of the needle. In the case of small diameter needles such as needles having an external diameter of less than 0.7 mm and an internal diameter of less than 0.45 mm (approximately 22 gauge or higher), this is a relatively complicated task when compared to the preceding steps.
The orifice may have to be created by laser drilling a hole at the correct point, by pressing a revolving disc cutter against the curved surface of the Huber needle, or by removing a quarter-cylinder length from the side of the shaft in the case of the Copen needle (no indication is given by Copen as to how one would go about creating the type of aperture disclosed). In all cases, the operations require careful positioning of the needle relative to the machining means, and a delicate machining of each and every needle. The magnitude of the cost differential between such needles and conventional needles arises from the differences between the complexity of the manufacturing processes involved in each type of needle.
It is an object of the present invention to provide a needle which has improved delivery characteristics when connected to an infusion system, and which can be manufactured cheaply and easily in large numbers at the same time unlike the prior art needles referred to above.
Accordingly, the invention provides a needle for the delivery of a liquid to a subject, comprising a shaft having an internal surface defining a longitudinally extending internal bore open at one end thereof to receive a liquid supply, the other end of the shaft being sharpened for penetration of the skin of the subject, the shaft being provided with at least one aperture in the form of a cut extending across the external surface of the side of the shaft at a sufficient depth to establish communication with the bore, wherein the external aperture area is greater than the internal aperture area, said areas being defined by the surface areas of the material removed between the external and internal surfaces of the shaft, respectively.
Because the or each aperture is in the form of a cut extending across the shaft, rather than a section removed along the length of the shaft or a hole drilled into or through the shaft (as in the prior art), the needle according to the present invention is far easier to manufacture. It is as easy to manufacture a large number of identical needles (by extending a cut across a plurality of shafts at the same time) as to form a cut in a single needle.
Additionally, the or each aperture thus formed has a larger external surface area than internal surface area, so that the needle delivers drug to a large area of tissue, thereby increasing the capability of absorption. The improved delivery characteristics of the needle will be illustrated below. In all needles where a hole is drilled into the surface of the needle, the external surface area (as defined above) is in fact less than the internal surface area due to the greater curvature of the internal surface.
Suitably, the sharpened end of the needle is in the form of a bevelled or conical tip and the aperture is formed by a linear cut made at an angle to the internal bore which is greater than or equal to the angle between the bevelled or conical surface and the longitudinal bore.
In a presently preferred embodiment, the cut extends in a direction approximately perpendicular to the longitudinal bore.
This allows the aperture(s) to be formed at exactly the same distance from the needle tip in a batch of needles by aligning the ends of a row of parallel adjacent needles and machining a groove or cut across the surface of the row, to form a series of identical apertures in the aligned needles.
Suitably, the width of the cut on the external surface of the shaft is greater than the width of the cut on the internal surface of the shaft. Preferably, the width of the cut on the external surface of the shaft is at least twice the width of the cut on the internal surface of the shaft.
It is preferred to maximise the external width of the cut relative to the internal width of the cut, so as to maximise the ratio between external and internal aperture areas.
According to a preferred embodiment, the shaft has a C-shaped cross section at the locus of the aperture, defined by the partial circumferences of the internal and external shaft surfaces and by the aperture surfaces formed by the cut which connect the internal and external shaft surfaces, said aperture surfaces being co-planar.
Such a cross-section can be obtained by manufacturing the needles as described in detail below.
Preferably, the shaft is provided with a plurality of said apertures, each of which is located a different distance from the sharpened end of the needle.
The provision of a plurality of apertures increases the area of tissue to which the needle can deliver. Additionally, the apertures can be spaced along the length of the shaft to provide more delivery sites. If a single aperture is provided on a needle, all of the tissue at the aperture may have poor absorption. By increasing the number of apertures, the probability that at least one aperture will deliver to a site with improved absorption is maximised.
Preferably, the ratio between the external aperture area and the internal aperture area is greater than or equal to the ratio between the external width of the shaft and the internal width of the bore. In general, this provides a substantially larger external aperture surface area.
Preferably, the depth of the cut relative to the external surface varies along the length of the cut. Further, preferably, the depth of the cut increases from zero at either end of the cut to a maximum depth towards the centre of the cut which is greater than or equal to the thickness of the wall of the shaft at that point.
In a preferred embodiment, the variation in the depth of the cut results from a planar cut surface machined across a curved external surface. Nevertheless, it is also possible to provide a curved machined cut across the external surface.
Suitably, the sharpened end is provided with a terminal orifice to assist in delivery of the liquid through the needle.
Preferably, the or each aperture is formed by the operation of a grinder on the surface of the shaft. Alternatively, the or each aperture is formed by another machining operation such as milling (although grinding is at present preferred due to lower costs and the fact that it is not as likely to give rise to difficulties such as chips being produced by the operation.
Preferably, the external and internal surfaces of the needle are defined generally in cross-section by two concentric circles. There is no strict necessity to use a circular needle with a circular bore, however, other than the fact that the tubing for such a needle is readily and cheaply available.
The invention also provides a method of manufacturing a plurality of delivery needles, comprising the steps of arranging a plurality of tubular shafts having internal bores in a row in parallel with one another and performing a machining operation across the row of shafts to create at least one aperture in the external surface of the side of each shaft at a sufficient depth to establish communication with the bore of each shaft.
It will be appreciated that this operation can be carried out on scores or hundreds of needles simultaneously. The manufacture of a batch of 200 needles according to the invention can be completed in only a few seconds. To carry out the manufacture of 200 of the prior art needles referred to above, on the other hand, would take 200 times the length of time required to carry out the machining operation for one needle, which does not include the additional time involved in positioning each successive needle in the correct position for machining.
Preferably, the machining operation includes running a grinder across the row of shafts to grind a line of grooves across the surfaces of the shafts, the grooves being sufficiently deep to provide communication between the external surfaces of the shafts and the internal bores of the shafts.
Optionally, the process also comprises the step of sharpening the shafts while they are arranged in a row.
In addition, the invention provides a liquid delivery device for delivering a liquid to a subject, comprising a reservoir for the liquid, means for driving the liquid from the reservoir to an outlet, and a delivery needle according to the invention in communication with the outlet.
Suitably, in such a device, the reservoir and driving means are located in a housing which is provided with means for attachment to the skin of the subject and wherein the delivery needle protrudes from the housing in use such that it penetrates the skin of the subject upon application of the device to allow delivery of the liquid through said needle.
Preferably, the delivery rate of the device is calibrated according to the pressure exerted by the driving means. By incorporating the needle according to the invention, such a device is freed from the variations and fluctuations in delivery rate which have otherwise been found to exist.
The invention will be further illustrated by the following description of embodiments thereof, given by way of example only with reference to the accompanying drawings.